Chytrid fungus

Chytridiomycosis is caused by two species of microscopic fungi Batrachochytrium dendrobatidis (“Bd”) and B. salamandrivorans (“Bsal”), which are often simply referred to as “chytrid”. Chytrid infection has been responsible for mass mortalities of amphibians with declines and extinctions in some species on six continents, including Europe.

Bd is primarily a parasite of amphibians but may also be able to infect some birds and crustaceans which live in freshwater habitats. A range of amphibian species have been infected by Bd in Europe, and with some serious declines observed in upland areas of Spain. Bsal has also been associated with mass die offs of wild European fire salamanders, leading to a 99% decline in the national population of this species in the Netherlands.

Studies have shown that, although all species of amphibian are susceptible to infection, some species appear to be tolerant , and do not develop disease symptoms. As a result, some non-native species such as African clawed toads, North American bullfrogs and alpine newts have been named as potential carriers or ‘vectors’. In the UK most native Natterjack toad populations which have been tested for Bd have tested positive, but are apparently unaffected. The effects of Bd on other UK species are less well understood. Although, in mainland Europe, Bd has been linked to mass mortalities of species such as the European midwife toad, in the UK no mass mortalities have been linked with chytrid.

Evidence suggests that chytrid causes death by preventing sufficient uptake of salts through the skin. Amphibians need salts for their circulation – without sodium, their hearts stop beating. Symptoms of infection in adult amphibians include reddening of the skin, excessive shedding of skin, skin ulceration (especially at the tips of the toes), abnormal posture, apparent “seizures” or unusual behaviours such as nocturnal species being active during the day. In most cases, however, there are no visible signs of disease and “apparently healthy” animals are often found dead. In adult fire salamanders, infection with Bsal leads to skin ulceration, listlessness and a lack of coordination. In tadpoles, the chytrid fungus only infects the mouthparts, and although this often causes no visible signs of disease, in severe cases it can retard growth and metamorphosis, and lead to death.

The infectious stage or ‘zoospore’ of chytrid can swim in water, though it is too small to be seen by the naked eye. It then embeds itself in the skin of an amphibian, where it reproduces before emerging back into water to infect new hosts. As a result animals may become infected by sharing a pond or waterbody, as well as by direct contact. In addition to transmission between animals, spores may persist in the environment in water or damp soil, and could therefore also be transmitted by birds, livestock and invertebrates; as well as by people, on boots or equipment such as pond nets and bottle traps.

Whilst amphibian chytridiomycosis may be treated in captivity, treatment in the wild is not currently possible. It is therefore essential that the spread of the disease to new areas or ponds is limited as far as possible by adopting a precautionary approach. ARG UK (2017) has produced an advice note for working with amphibians, including the disinfection of footwear and equipment between ponds at different sites. In addition, although Bsal has not been identified in wild populations Great Britain, great care should be taken with handling and keeping imported or captive amphibians to ensure that there is no possibility of disease spread.